The present invention relates to piezoelectric transformers utilizing the piezoelectric effect and more particularly to improvements in and relating to piezoelectric transformers well suited for use with inverters for liquid crystal displays, high voltage generating circuits for dust collectors and the like.
Recently, piezoelectric transformers are frequently used as back light inverter transformers for liquid crystal displays of lap-type or notebook-type personal computers and the like. Heretofore known piezoelectric transformers have been so designed that there exists between a driving or operating frequency and the length of a baked element a certain relation which satisfies the desired resonance condition and therefore a xcex driving or xcex/2 driving constitutes a basis if the wavelength corresponding to the operating frequency is represented by xcex Al so, generally known constructions include a so-called Rosenberg type in which one end of the piezoelectric element serves as an input portion and the other end serves as an output portion and a so-called central driving type in which the both ends serve as output portions.
Referring to FIG. 11, there is illustrated the basic construction of a Rosenberg type piezoelectric transformer and the manner of its vibration. In the case of FIG. 1(A), a primary-side electrode 102 is provided on each of the upper and lower surfaces of the left portion of a piezoelectric element 100 in plate form and a secondary-side electrode 104 is provided on the left end face of the piezoelectric element 100. The left side of the piezoelectric element 100 is polarized in the thickness direction and the right side of the piezoelectric element 100 is polarized in the lengthwise direction. When such Rosenberg type piezoelectric transformer is subjected to the xcex driving, the piezoelectric element 100 vibrates as shown by a dotted line in FIG. 1(B). Also, when subjected to the xcex/2 driving, the piezoelectric element 100 vibrates as shown by a dotted line in FIG. 1(C).
However, the foregoing background art has the following disadvantages.
(1) Where such piezoelectric transformer is used as a back light inverter transformer of a liquid crystal display, lead wires are usually used for a connection from the inverter transformer to the cold-cathode tube of the inverter transformer. Since a stray capacity exists in the lead wires, any increase in the operating frequency increases the loss due to the stray capacity. There is another disadvantage that the stray capacity in the lead wires has an effect on the impedance on the secondary side of the inverter transformer thus failing to obtain a sufficient output voltage. As the means of avoiding this, there are two methods: {circle around (1+L )} a method of shortening the lead wires as far as possible to reduce the absolute value of the stray capacity, and {circle around (2+L )} a method of lowering the operating frequency to prevent any decrease in the impedance of the lead wires.
Then, the actual circumstances in the field of liquid crystal displays are such that there is the demand for further decrease in the thickness of displays. On the other hand, if a piezoelectric transformer is mounted on the back side or the lateral side of a liquid crystal display, the desired reduction in thickness is inevitably rendered difficult. Therefore, it is still necessary to mount an inverter in the vicinity of a main circuit and this makes difficult the shortening of the lead wires in the above-mentioned {circle around (1+L )}. In view of these situations, it is desirable to resort to the way of preventing the decrease in the impedance of the lead wires by the method of lowering the operating frequency in the above-mentioned {circle around (2+L )}. However, even if the operating frequency is set to a lower value, to set the operating frequency in the audible frequency range of man or pets is not desirable from the noise generation point of view. Thus, it is desirable to employ an operating frequency of higher than the audible frequency range or about 60 kHz or so.
However, if the previously mentioned xcex driving or xcex/2 driving is effected at such low frequency, the overall length of a piezoelectric element is inevitably increased. For example, the required length of a piezoelectric element for operating at 60 kHz is about 55 mm in the case of xcex driving and about 28 mm in the case of xcex/2 driving. It is to be noted that while the element length corresponding to an operating frequency varies depending on the elastic constant of a material, the change in elastic constant is small if a ceramic material is used. Thus, there is a limitation to the variation in the element length owing to the selection of material.
(2) Where the operating frequency is decreased, the length of a piezoelectric element can be decreased to a greater extent by the xcex/2 driving system than by the xcex driving system. However, as shown in FIG. 11(C), the xcex/2 driving of the Rosenberg type has the node of vibration at a single place and thus there is a problem for the support of the piezoelectric element. In addition, a high voltage is produced at the output end of the piezoelectric element so that the arrangement of any other electronic components is not desirable from the standpoint of insulation and the like. On the other hand, in the case of a center driving type in which electrodes are provided in the central portion of a piezoelectric element and an output is produced from the both ends, a high voltage is generated at each end so that it is impossible to arrange any electronic components in the vicinity of the ends and this results in increase in the component mounting areas at the ends of the piezoelectric element thereby preventing the desired reduction in the size of liquid crystal displays.
(3) There are liquid crystal displays of various sizes and the size of the cold-cathode tube of each display differs depending on its size. As a result, it is necessary to modify the specification of the inverter transformer so that an output power corresponding to the size of the cold-cathode tube is obtained. However, this deteriorates the productivity with the resulting increase in cost.
Noting the foregoing deficiencies in the prior art, it is an object of the present invention to provide a piezoelectric transformer so designed that it is operable at a low frequency, is made smaller and more compact, is reduced in mounting area and is improved in productivity.
To accomplish the above object, a Rosenberg-type piezoelectric transformer according to the present invention features that the length of its element in the direction of transmission of mechanical vibrations is selected to be about {fraction (1/4+L )} of the wavelength of the operating frequency.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings.